Method of improving anti-corrosion characteristics of anodized aluminum

ABSTRACT

The invention relates to an improved anodizing process that by modifying the reagents used in a sealing step of the anodizing process may be used to provide for improved anti-corrosion properties of the finished anodized part.

FIELD OF INVENTION

The present invention relates to an anodizing process and in particular to an improved anodizing process wherein the resultant anodized material has improved anti-corrosion characteristics.

BACKGROUND

Aluminum use is widespread. The properties of the metal, specifically its light weight, durability and strength, make it ideal for a variety of applications. These include those in the automotive industry where it is used for components such as trims and moldings, the housing industry where it is used for windows and the pharmaceutical industry where is used in processing reactors and storage containers.

In all these environments exposed aluminum is vulnerable to attack by corrosive elements in the ambient atmosphere. It is therefore known and conventional practice to subject aluminum to an anodizing process, which is an electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. This coating is desirable due to the fact that the anodized part has increased corrosion resistance, increased durability/wear resistance and also in certain applications may be suitable for coloring during dying. This is particularly useful for use in the fabrication of automobile trims and moldings where the aluminum is used as a decorative feature on the outer body of the automobile.

The step of anodizing is well known to the person skilled in the art. It consists of creating an electrical circuit in an acidic solution. A cathode is connected to the negative terminal of a voltage source and placed in the solution. An aluminum component is connected to the positive terminal of the voltage source and also place in the solution. When the circuit is turned on the oxygen in the anodizing solution will be liberated from the water molecules and combine with the aluminum on the part forming an aluminum oxide coating. The resultant part will have improved characteristics relative to the un-anodized part.

In production facilities, however, the actual anodizing step is only one step in the anodizing process which includes a plurality of additional steps including those of cleaning, rinsing, etching, de-smutting and deoxidizing and sealing the anodic coatings. The purpose and makeup of each of these additional steps are well known to the person skilled in the art, but for the sake of explanation the main benefits of each will be highlighted here. It will be understood that each of the individual steps may be repeated one or more times throughout the process:

Cleaning. Cleaning provides for a removal of contaminants such as metal working lubricants and the like from the surface of the metal and prepares the surface for further processing.

Rinsing. This is used to terminate any reaction process that has immediately preceded the rinsing step. This step also provides for a removal of unwanted contaminants from the preceding step and ensures a uniformity of chemical reaction in subsequent steps. Typically multiple rinsing steps will be used with each rinsing step being used to prevent cross contamination between reagents of sequential steps.

Etching. Etching is useful in developing a smooth uniform finish.

Deoxidizing & Desmutting. Provide for an activation of the aluminum surface prior to actual anodizing and removes any residual surface oxides from preceding steps.

Sealing Anodic Coatings. The purpose of sealing an anodic coating is to close the pore structure of the anodic and render the film inert. This is accomplished by chemically changing the surface structure through hydration and may be effected using a cold seal and/or a hot seal. These sealing processes differ in the temperature of the seal and the metallic based sealing compound used. Using such a sealing process renders the surface of the aluminum as a non-staining, non-absorbing and non-reactive surface, ideal for use in the environments where aluminum often finds itself used. Once an anodized part has been through the sealing steps it is considered to be fully sealed.

Despite the benefits of these steps and the improved properties of the aluminum post processing there is always a need to improve the material further. Specific testing of the processed part includes the exposure of the part to a caustic solution, often having a pH greater than 10.5 and typically at a pH of about 12.5 for a time period in the range 10 or 16 minutes depending on the specifics of the test being conducted. In a processing facility it is necessary for the anodized part to pass an immersion test in such solutions prior to it being acceptable for use in the end application. The appearance of the tested material is then compared against a known standard and the material given a result against that standard. One typical test uses the following: Grade 0 No visible impact or discoloration of the surface. Grade 1 Very slight discoloration. Grade 2 Surface is partly discolored, milky Grade 3 The entire surface is discolored, milky. Grade 4 The surface is strongly affected by the solution.

For a test to have a requisite level of anti-corrosion characteristics a Grade 0 is optimum. This then represents a goal for all anodizing processes, and is one that is not always achieved.

For these reasons there is a need to provide for improvements to the known anodizing techniques.

SUMMARY

These and other problems are addressed by a process in accordance with the teaching of the invention which incorporates the use of a silica within the sealing steps effected subsequent to the anodizing of the aluminum. The use of the silica provides for improved anti-corrosion properties over those materials not treated with the silica sealant. These anti-corrosion properties are particularly evidenced by the tolerance of the anodized part to exposure to caustic solutions having a pH greater than 10.5.

Accordingly the invention provides a method according to claim 1 with advantageous embodiments provided in the dependent claims thereto. The invention also provides an aluminum part according to claim 7 with advantageous embodiments in the dependent claims thereto. Use of silica in accordance with claim 10 is also provided.

These and other features will be better understood with reference to the following figure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawing in which:

FIG. 1 is a flow sequence illustrating steps utilized in an anodizing process according to the teaching of the invention.

FIG. 2 is a scanning electron microscopy (SEM) image of a part treated in accordance with the teaching of the invention.

FIG. 3 is another scanning electron microscopy (SEM) image of a part treated in accordance with the teaching of the invention, at a different level of magnification.

FIG. 4 is a scanning electron microscopy (SEM) image of a part not treated for comparison purposes.

FIG. 5 is similar to FIG. 4, yet at a different level of magnification.

FIG. 6 is an EMX spectra showing the presence of silica on a part treated in accordance with the teaching of the invention.

FIG. 7 is another EDX spectra for a treated part.

FIG. 8 is an EDX spectra for a non treated part where the absence of silica is evident.

FIG. 9 is another EDX spectra fro a non-treated part showing the absence of silica on the surface.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to exemplary embodiments and with reference to FIGS. 1 to 10.

As shown in FIG. 1, an anodizing process according to the teaching of the invention incorporates many of the conventional and known steps which are commonly used in conventional anodizing process including cleaning (Step 100), etching (Step 110), desmutting/deoxidizing (Step 120), the actual anodizing of the metal (Step 130), the application of cold and hot rinses (Steps 140, 150) and then the subsequent drying (Step 160). As was highlighted above in the background section between one or more of these steps may be including a rinsing step (Step 170) which serves to terminate the reaction from the preceding step and prevent the occurrence of cross-contamination of reagents from sequential steps. The benefits of these steps are well known and will not be described herein save to say that the order of the cold and hot seals are not important bar that they should be applied post anodizing to ensure that the surface of the anodized part is adequately sealed.

As mentioned above the sealing processes include the purpose of sealing an anodic coating is to close the pore structure of the anodic and render the film inert. This is accomplished by chemically changing the surface structure through hydration and may be effected using a cold seal and/or a hot seal. These processes differ in the temperature of the seal and the metallic based sealing compound. By way of example a cold seal may typically be effected using a combination product containing nickel and cobalt salts and auxiliary substances containing fluoride. Such a seal is applied in a slightly acidic solution, pH in the range 5-7, and the temperature of the bath about 30 degrees centigrade. Known suppliers of such chemical sealing agents include Nabu Oberflächentechnik GmbH and Henkel Surface Technologies.

By way of contrast, a hot seal is effected using temperatures greater than 60 degrees centigrade and typically 90-100 degrees. The solution is however still slightly acidic. The chemical product used in the hot seal process prevents film formation when anodized aluminum is immersed with a range of surfactants used to aid de-wetting of the parts. Again, the two companies identified above Nabu Oberflächentechnik GmbH and Henkel Surface Technologies provide such chemical seals.

The present invention provides for the addition of an additional step which may be applied as part of the sealing process. While it may be incorporated within the cold seal process it is preferable that it is incorporated within the hot seal, but in any case before the anodized aluminum is fully sealed. According to the teaching of the invention, the sealing solution used is modified to include silica, i.e. the silica is provided in an aqueous solution. This solution of the conventional hot seal, modified to include silica, is used within the bath where the anodized part is immersed. When used as part of the hot seal step the hot seal serves its normal purpose, i.e. it is used to prevent film formation when anodized aluminum is hot water sealed with the range of surfactants used to aid de-wetting of the parts. The silica solution represents an additional reagent added to the hot seal solution to increase the corrosion resistance of the part at elevated caustic solutions. This hot seal solution, modified according to the teaching of the invention typically includes a minimum of about 1% silica, and desirably a volume in the range 3-8% and optimally 5%. The temperature of hot seal solution should be typically greater than about 65 degrees centigrade with optimum performance reached with temperatures of about 90 degrees centigrade plus/minus 5 degrees. Desirably the silica is provided in a SiO₂ form and more preferably as a metal silicate such as an alkaline metal silicate.

When subjected to the typical testing arrangement involving an alkaline test, the completed alkaline detergent tests showed parts subjected to the testing procedure having a passing Grade 0. Accelerated Corrosion Tests completed showed visually the improvement from a corroded (etched) surface to a clear non-corroded surface.

Analysis of aluminum parts treated in accordance with the teaching of the present invention showed marked differences with parts treated using convention processes. FIGS. 2 and 3 show the results of scanning electron microscope images taken of a surface treated while FIGS. 4 and 5 show corresponding non-treated parts. It is evident that the pores that are normally present on the surface, of sizes of about 10 microns, are blocked in the parts of FIGS. 2 and 3. When one examines EDX spectra (FIGS. 6 and 7 for the treated part, FIGS. 8 and 9 for the non-treated) it is evident that there is a presence of silica on the treated surface whereas on the non-treated surface there is none.

While it is believed that the silica on the surface of the treated part fills each of the pores, it will be appreciated that the invention should not be limited to such an arrangement as any arrangement that provides silica on the surface of a treated part irrespective of whether a pore is filled completely or otherwise will be considered as falling within the scope of the claimed invention.

Therefore it will be understood that by incorporating silica as part of the sealing steps post anodizing of the aluminum that the silica causes a blocking of the pores that are normally present on the surface of the treated parts. This blocking of the pores serves to reduce the surface area that is exposed to attack, and therefore improves the anti-corrosion characteristics of the treated part. As a result, parts that have silica on the surface—which is provided by processing these parts according to the teaching of the invention—may be considered better parts for use in environments where they may be exposed to corrosive environments. Such environments include those of the automotive industry where aluminum parts are used for body components such as trims or mouldings, the glazing industry where anodized aluminum is used for window frames and the medical devices or pharmaceutical sectors where it is essential that fabricated devices can be used without fear of corrosion.

While the invention has been described with reference to the broad term silica, it will be understood that silica may be provided in any one of a number of different forms and or variants and it is intended that the invention not be limited to any one specific example.

The present invention is based on a realization by the inventors that the inclusion of a silica based solution within a sealing step during the anodizing process serves to increase the anti-corrosion characteristics of the anodized part.

Using a process such as that outlined above leads to the provision of anodized aluminum parts that are extremely tolerant towards hostile environments such as those experienced by components in outside conditions such as automotive components and building materials such as window frames etc. or indeed in medical devices where the part requires an excellent stability.

While the invention has been described with reference to specific examples it will be understood that these are provided as exemplary of the application of the invention and it is not intended that the invention should be limited in any way except as may be deemed necessary in the light of the appended claims. Furthermore, the ordering of the steps highlighted for illustrating the anodizing process are not to be considered as sequential except where specifically indicated. For these reasons and others, modifications to the exemplary embodiment highlighted here are intended to be encompassed within the spirit and scope of the present invention.

The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 

1. A method of providing an anodized aluminum part, the method including the steps of: a) Anodizing the aluminum part, b) Sealing the anodized part using a sealing solution, and wherein the sealing solution includes aqueous silica.
 2. The method as claimed in claim 1 wherein the sealing solution includes at least 1% silica.
 3. The method as claimed in claim 1 wherein the sealing solution includes silica in the range 3-8% and preferably 5%.
 4. The method as claimed in claim 1 wherein the sealing solution is a hot seal, the sealing being conducted at a temperature greater than 65 degrees centigrade and optimally at a temperature in the range 85 to 95 degrees centigrade.
 5. The method as claimed in claim 1 wherein the silica is provided as an aqueous silicate solution, optimally in the form includes of a metal silicate.
 6. The method as claimed in claim 1 when used in the preparation of automotive parts.
 7. An aluminum part treated in accordance with claim 1, the part being fully sealed and having silica on its surface.
 8. The part as claimed in claim 8 wherein pores on the surface are not greater than 5 microns in diameter.
 9. The part as claimed in claim 7 or 8 wherein when the part is exposed to a corrosive caustic solution having a pH greater than 12.5 for a time period greater than 10 minutes, the part shows no visible impact or discoloration of the surface
 10. Use of silica in a sealing process for anodized aluminum in the preparation of parts for the automotive industry. 